Fatigue Behaviour of Medium Carbon Steel of Different Grain Structures

Transcription

1 Fatigue Behaviour of Medium Carbon Steel of Different Grain Structures S. AbdulKareem 1* S. I. Talabi 2 J. A. Adebisi 2 T. Yahaya 2 O. H. Amuda 3 1.Department of Mechanical Engineering, University of Ilorin, Nigeria 2.Department of Materials and Metallurgical Engineering, University of Ilorin, Nigeria 3.Department of Metallurgical and Materials Engineering, University of Lagos, Nigeria Abstract This paper investigated the effect of heat treatment operations on the fatigue resistance of low carbon steel. Specimens after preparation for fatigue testing were subjected to annealing, normalizing and quenching heat treatment. Results show that the annealed specimen had the largest number of cycles to failure, indicating a high fatigue resistance. The microstructure of the specimens was examined in other to corroborate the obtained property with the microstructure. When compared with the untreated specimen, the annealed specimen (with optimum fatigue resistance) shows a large grains size of pearlite which was distributed across the entire surface of the microstructure. Generally, it was found that the size and distribution of specimens grains affect the resistance of the low carbon steel to fatigue failure. Keywords: fatigue, low carbon steel, heat treatment, microstructure 1. Introduction Majority of engineering designs, structures, machines, and systems involve materials analysis, selection and categorization. These engineering materials are numerous and among them are metals, composites, ceramics and plastics. The most useful one being metals due to their mechanical properties like tensile and compressive strength, ductility, brittleness, fatigue strength and others. Among these metals, low carbon steel has a very high status in terms of its versatility, cheapness, ease and low cost of production which makes it the most widely used metallic material. The mechanical properties (and some other properties like electrical, chemical) form the major characterization and categorization of metals and also determines the type of material needed for certain specific engineering purposes. Hence due to the suitability of metals for very wide range of applications, the need to study and establish certain facts regarding metals arises. Metallic materials consist of a microstructure of small crystals called grains or crystallites. The grain sizes and composition which make up the nature of the grains are one of the most effective factors that determine the overall mechanical behaviors (hardness, tensile strength, toughness, wear resistance) of metals (Qamar, 2009). An efficient way to manipulate the properties of metals via grain structure manipulation by controlling the rate of diffusion and that of cooling is that of heat treating procedures. In other words, heat treatment systematically alters the microstructural arrangements and sizes of the metals grains which are the reasons for the changes observed in mechanical properties of metals after heat treatment operations (Totten, 2007). Some properties of the steel might be required to be varied due to design specifications and requirements. This is mostly done by various heat treatment operations (Daris and Oelmann, 1983).These heat treatments give the desired properties of carbon steel, but sometimes at the expense of some other properties. Hence, before selecting a type of heat treatment, various factors need to be considered including the type of loading the materials will be subjected to in the service conditions. Of the various loading types is fatigue loading which is a load whose magnitude varies with (or is a function of) time. Hence, using fatigue loading as a case study, it would be observed that different materials fail at different load cycles and this character can be directly traced to the different grain structures, sizes and orientation of the materials. 2. Materials and Method The chemical composition of the as-received specimen is presented in Table 1. The specimens were prepared for fatigue test by machining on a lathe machine. The specimens were then subjected to annealing, normalising and quenching. In the annealing operation specimen were heated to an austenizing temperature of 850 C±5 C, soaked for 20 minutes in order to attain uniform heat distribution and left to cool in the muffle furnace (Power 5kW, Phase 1, Weight 55kg, Temperature 1200 o C, Voltage 220V, Dimension of chamber 300x200x120mm). Similar operation was carried out during normalizing and quenching operation except that the specimens after heating were cooled in air and quenched in silica sand respectively. Fatigue test was done with is an Avery Denison type with serial no Grips are provided for torsion tests and for bend tests on flat specimen. Standard microstructural test pieces (as-received and heat treated) were prepared and ground using emery paper with grit 220 to 600 microns in succession. The ground surface was polished using a mixture of alumina and diamond paste and then etched in a solution containing 2 ml Nitric acid and 100 ml Ethanol. The etched surfaces were left for 20 seconds before they were rinsed and dried. The specimens crystals morphology 10

2 was viewed under an optical metallographic microscope at 600X magnification and the photomicrographs are shown in Plates1-4. Table 1. Composition of specimen Elements % wt Composition Iron Carbon Silicon Manganese Phosphorous Chromium Nickel Copper Vanadium Result and Discussion 3.1 Effect of Heat Treatment on the Microstructure of Specimens Plate 1 shows a uniform distribution of ferrite which are the brightly coloured parts and pearlite (a mixture of ferrite and cementite) which are the dark or black spots and which are congested along the grain boundaries and also slightly dispersed over the ferrite. The grain sizes are moderate and the boundaries appear smooth, apart from the pearlitic presence. Plate 1: Micrograph Of as-received specimen at X600 Plate 2 is the annealed specimen and the micrograph shows a large grain size of pearlite which was distributed across the microstructure. Unlike the untreated specimen, the pearlite phase which was congested along the grain boundaries is now distributed across the entire surface of the microstructure. The size of the grains makes it easy for dislocation to glide during fatigue loading, hence the high fatigue resistance. Annealed materials are usually ductile and stress free due to these stated characteristics. In addition, when a crack or void forms in a pearlitic matrix, it will tend to run along the length of a pearlite lamella. Examining thistype of fracture under the SEM reveals that the base of the dimples contain fractured pearlite lamella (Scott, 2008) 11

3 Industrial Engineering Letters Plate 2: Micrograph of Annealed Specimen at X600 When compared to the annealed sample, the micrograph of the normalized sample (Plate 3) shows a relatively smaller grain size and a balanced distribution of ferrite and pearlite with uniform fine grain and quite similar to the control specimen, but appears relatively better than annealed and control samples Plate 3: Micrograph of normalised specimen at X600 In addition, the sand cooled specimen shows a fine and relatively smaller ferrite and pearlite distribution and has a relatively lower amount of pearlite concentration as observed in the control and annealed specimen and it is observed that the grain boundaries appear thin and congested (Plate 4). 12

4 Plate 4: Micrograph of silica quenched specimen at X Effect of Heat Treatment on the Fatigue Property of Specimens As seen from the results from graphs (Figure 1-4), a significant variation in the no. of cycles to failure was observed with respect to all the tested specimens. The annealed specimen had the largest number of cycles to failure, indicating a high fatigue resistance, followed by the specimen that was quenched in silica sand and then the normalized specimen. This agrees with the microstructures of the annealed specimen has it exhibit large and coarse distribution of pearlite (Senthilkumar and Ajiboye, 2012). Figure 1: S-N curve for the as-received specimen Figure 2: S-N curve for the annealed specimen 13

5 Figure 3: S-N curve for the normalized specimen Figure 4: S-N curve for the quenched specimen 4. Conclusion Heat treatment operation can be used to optimize the fatigue resistance of low carbon steel. The annealed specimen had the largest number of cycles to failure, indicating a high fatigue resistance, followed by the specimen that was quenched in silica sand and then the normalized specimen. The microstructure of the specimens corroborate with the obtained properties. The fatigue behaviour of the specimens was attributed to grain size and distribution. During application in which fatigue property is of optimum importance, annealed specimen which shows a better fatigue resistance is recommended. References 1. Daris D.J. and Oelmann L.A. (1983), The structures, properties and Heat Treatment of metals. The Pitman Press, Great Britain, 2. Scott MacKenzie (2008), Houghton International, Inc. Overview of the Mechanisms of Failure in Heat Treated Steel Components. ASM International: Failure Analysis of Heat Treated Steel Component, pp Totten G.E. (2007) (ed). Steel Heat Treatment Handbook. vol. 1. Metallurgy and Technologies, vol. 2, Equipment and Process Design, 2nd edition. CRC Press, Boca Raton. 4. Senthilkumar T., and Ajiboye T. K. (2012), Effect of Heat Treatment Processes on the Mechanical Properties of Medium Carbon Steel. Journal of Minerals & Materials Characterization & Engineering, Vol. 11, No.2, pp Qamar S.Z. (2009), Effect of Heat Treatment on Mechanical properties of H11 tool steel. Journal of Achive Materials and Manufacture Engineering, vol. 35, No. 2, pp Heat Treatment of Steel (1924), Application and Heat Treatment of Society of Automobile Engineer, Carbon and alloy Steels. Machinery s Hand Book. 14

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